The patents referenced above describe mechanisms and apparatus suitable for analyzing specimens for specific microorganisms utilizing a plastic tray or card which contains a plurality of dried culture media specific to a single genus or species of organism. The dried media are contained in separate cells in the card which are connected by a network of passageways to a filling port. When a fluid sample is inserted into the card, mixed with media in the cells, and incubated, the organisms present in the specimen interact with the specific culture media. The interaction of the specimen and the specific culture media produces characteristic optical changes in the contents of the cell which are read to indicate presence of the organisms. The optical change in each cell involves a change in light transmitting properties thereof either through a color change or a change in turbidity. The optical change usually is caused by the metabolic activity of the organism which, for example, may produce an acid which changes the color of a pH sensitive indicator in the media. The change and the light transmitting properties of the medium also can be caused by a precipitate forming in the medium due to metabolic activity of the organism or by the mass of growing colonies of the organism. The metabolically caused changes generally yield considerably earlier results than do growth caused changes. The specific media designed for use in the cards of the aforesaid system all are designed to favor growth of one genus or species of microorganism and to inhibit growth of other organisms. The media are capable of being freeze dried and they are formulated to function in the low oxygen environment of the cells of the card described in detail in the above referenced patents.
Salmonella spp. are microorganisms which are pathogenic to the human body. The presence of these microorganisms in the urine, feces, or blood of the human body is a reliable indication of bacterial infection in the body. Salmonella can be introduced into a human body through contaminated food and water. Therefore, it is desirable to detect, identify and enumerate Salmonella in the effluent from sewage treatment plants, in food produced by food processing plants, meat packaging plants and hospitals to insure the wholesomeness of products intended for consumption by humans. The above concerns also apply as well to animals other than humans.
Heretofore, media which could selectively isolate Salmonella in pure cultures of the bacteria have included Salmonella Shigella Agar, Bismuth Sulfite Agar and XLD Agar all produced by BBL (Baltimore Biological Laboratory) and the HEA formulation disclosed in the text, "Manual of Clinical Microbiology," 2nd Edition. All rely on the non-utilization of certain carbohydrates and therefore produce unwanted false positives especially in mixed culture media for use in machines as described in the patents referenced above.